1. Field of the Invention
This invention relates to the refining of molten aluminum. More particularly, it relates to the dispersion of a gas in the molten aluminum.
2. Description of the Prior Art
Molten aluminum, as derived from most common sources, such as primary metal, scrap and re-melt ingot, usually must be purified before being cast into ingots, sheets or bars. This may be done by bubbling an inert gas, i.e. nitrogen or argon, through the aluminum in molten form. In some embodiments, a halogen gas, usually chlorine, is added, or the halogen gas may be used alone for such purification purposes. This type of treatment can remove dissolved hydrogen, alkali metals such as sodium and lithium, and small solid particles such as aluminum oxide. The effectiveness of a given volume of gas in such treatment is increased by reducing the bubble size of the gas in the molten aluminum, thereby increasing the total gas-metal surface area. The effectiveness of the gas bubbles is also increased by the dispersing of said gas bubbles throughout the body of molten aluminum to be treated. One very effective way of both making small bubbles and dispersing them is by the use of a spinning nozzle positioned in the body of molten aluminum. Commercial systems are available for this purpose, including the SNIF.TM. systems of Union Carbide Industrial Gases Inc. for in-line refining of metals flowing from a holder to a casting station. The Pelton patent, U.S. Pat. No. 4,784,374, discloses and illustrates a particular embodiment of said SNIF.TM. system.
The refining rate of such a spinning nozzle system can be increased by increasing the process gas flow rate employed therein. It is usually also necessary to increase the nozzle rotating speed to continue the desired making of small bubbles and the dispersing of said small bubbles throughout the molten aluminum in the refining zone of the system. Such increase in gas flow and nozzle rotating speed are usually accompanied by increased turbulence on the surface of the molten aluminum. The maximum refining rate of a given refining system, however, is limited by the maximum surface turbulence or roughness that can be tolerated therein.
Excessive surface turbulence is undesirable in a refining system for several reasons. Thus, the increased metal surface area that is produced thereby leads to higher reaction rates with any reactive gas that might be present. For example, oxygen from air will react to form aluminum oxide films, and water vapor from the air will react to form hydrogen in the metal and oxide films. Furthermore, when solid particles are carried to the molten metal surface by the refining gas bubbles, surface turbulence may interfere with their desired separation from the bubbles and their incorporation into the floating dross layer formed over the body of molten aluminum. Excessive turbulence may also cause floating dross to be re-dispersed into the molten aluminum. While the quantitative effects of excessive surface turbulence are difficult to measure, those skilled in the aluminum refining art are nevertheless aware from experience that high surface turbulence is undesirable, and will strive to limit such surface turbulence to levels considered acceptable in practical commercial operations.
There is a need and desire in the art to increase the aluminum refining rate of spinning nozzle systems. Thus, it is desired to increase gas flow rates and nozzle rotating speeds so as to increase the maximum useful rate of refining without the onset of excessive surface turbulence as presently encountered in such spinning nozzle systems.
It is an object of the invention, therefore, to provide an improved refining system for the production of aluminum.
It is another object of the invention to provide an aluminum refining system employing one or more spinning nozzles and capable of operating at enhanced refining rates.
It is a further object of the invention to provide a spinning nozzle aluminum refining system capable of operating at higher gas flow rates and nozzle rotating speeds without a corresponding increase in turbulence on the surface of the molten metal.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.